全文获取类型
收费全文 | 504534篇 |
免费 | 69261篇 |
国内免费 | 1242篇 |
专业分类
575037篇 |
出版年
2019年 | 5976篇 |
2018年 | 7084篇 |
2017年 | 7071篇 |
2016年 | 8431篇 |
2015年 | 10476篇 |
2014年 | 11435篇 |
2013年 | 14943篇 |
2012年 | 14387篇 |
2011年 | 14009篇 |
2010年 | 11113篇 |
2009年 | 9578篇 |
2008年 | 11612篇 |
2007年 | 11716篇 |
2006年 | 11186篇 |
2005年 | 10807篇 |
2004年 | 10590篇 |
2003年 | 10231篇 |
2002年 | 10127篇 |
2001年 | 24562篇 |
2000年 | 24961篇 |
1999年 | 19694篇 |
1998年 | 6146篇 |
1997年 | 6534篇 |
1996年 | 6050篇 |
1995年 | 5737篇 |
1994年 | 5567篇 |
1993年 | 5549篇 |
1992年 | 15436篇 |
1991年 | 14766篇 |
1990年 | 14096篇 |
1989年 | 13789篇 |
1988年 | 12907篇 |
1987年 | 12105篇 |
1986年 | 11389篇 |
1985年 | 11439篇 |
1984年 | 9291篇 |
1983年 | 8025篇 |
1982年 | 6290篇 |
1981年 | 5751篇 |
1979年 | 9025篇 |
1978年 | 7232篇 |
1977年 | 6604篇 |
1976年 | 6158篇 |
1975年 | 6903篇 |
1974年 | 7578篇 |
1973年 | 7541篇 |
1972年 | 6951篇 |
1971年 | 6279篇 |
1970年 | 5454篇 |
1969年 | 5340篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
982.
The effect of methyl bromide (MB) was tested on active and anhydrobiotic Aphelenchus avenae. A. avenae was induced into anhydrobiosis by three different techniques. Both active and anhydrobiotic nematodes were subjected to 3,000 μ1 MB/liter air for 14 periods from 0 to 82 h. Anhydrobiotic nematodes were more resistant to fumigation than active nematodes, regardless of the technique used to induce anhydrobiosis. The percent survival decreased with increasing MB exposures (μ1 MB × h). For an LD₉₅ of 45,000-54,000 μ1/1 × h were required for active nematodes and >279,000 μ1/1 × h for anhydrobiotic nematodes. 相似文献
983.
984.
S. C. Ng A. H. Sathananthan P. C. Wong S. S. Ratnam J. Ho H. Mok M. N. Lee 《Molecular reproduction and development》1988,19(3):253-263
Previous studies by a French group (Fertil Steril 44:645–651, 1985) have shown that two-to eight-cell human embryos can survive slow freeze-thawing with propanediol in a biological freezer. These embryos were assessed for morphological appearance by phase-contrast microscopy. We assessed the structure of 25 frozen-thawed one- to 12-cell embryos, obtained from our in vitro fertilization (IVF) and GIFT programmes, by phase-contrast and electron microscopy, using the same method of cryopreservation. One-fourth of the embryos examined had all cells intact, and more than one-half the embryos had over 50% of their cells well preserved. Some of these embryos had unequal blastomeres and cytoplasmic fragments. Ultrastructural assessment revealed good preservation of fine structure in the intact blastomeres of all embryos and maintenance of cell-to-cell contacts. Most cytoplasmic organelles, cell membranes, and nuclei were well preserved compared to nonfrozen controls. The cells that were cryoinjured showed varying degrees of disorganization of the cell membrane, cytosol, and cellular membranes, including swelling and disruption of the nuclear envelope. Disruption of the zona was somewhat rare. Small cytoplasmic fragments were less prone to cryoinjury than blastomeres. The use of propanediol for embryo cryopreservation seems to be feasible; frozen embryos with more than 50% cells intact have produced 10 pregnancies after embryo transfer (Fertil Steril 46:268–272, 1986). Replacement of 17 frozen embryos in seven patients has resulted in a twin pregnancy in Singapore. However, the effects of freezing on the mitotic spindles of embryonic cells need to be investigated further. 相似文献
985.
986.
The determination of purine levels in human and mouse plasma 总被引:2,自引:0,他引:2
Variable levels of acetic anhydride have been recommended for addition to one of two reagents used in the glyoxylic acid method for the determination of tryptophan. For use of this reagent immediately after preparation it was shown that a minimum of 16% (v/v) of acetic anhydride should be included in the formulation to obtain near-maximum sensitivity. It was further demonstrated that reagent formulations with and without acetic anhydride changed with exposure to light. The observed changes are manifest as changes in the relative sensitivities of the assay. Several modifications are recommended to improve the sensitivity and stability of the acetic anhydride-containing reagent in this assay. 相似文献
987.
988.
Core temperature decreases throughout short-term maximal exercise in heart-failure patients. To investigate possible causes for this unusual response to exercise, we studied core (pulmonary arterial blood), femoral vein, muscle, and skin temperatures in eight patients with severe heart failure who performed maximal upright incremental bicycle exercise to 50 W. A normal group (n = 4) was exercised for comparison. In the heart-failure patients, core temperature was 36.95 +/- 0.37 degrees C at rest, significantly (P less than 0.05) decreased at 25 W of exercise to 36.59 +/- 0.40 degrees C, and at 50 W remained decreased to 36.57 +/- 0.40 degrees C. In comparison, we found that the resting core temperature in the normal subjects was 37.28 +/- 0.34 degrees C, was the same at 25 W (37.29 +/- 0.41 degrees C), and increased significantly (P less than 0.05) to 37.50 +/- 0.32 degrees C at 50 W of exercise. Femoral vein temperature in heart-failure patients (n = 6) was below core temperature throughout exercise to 25 and 50 W (36.22 +/- 0.62 and 36.34 +/- 0.65 degrees C, respectively). Muscle temperature (n = 7) was significantly (P less than 0.05) lower in the heart-failure patients (34.8 +/- 1.1 degrees C) at rest compared with the normal subjects (36.2 +/- 1.0 degrees C). During exercise, muscle temperature increased above core temperature in only four of the heart-failure patients and was significantly (P less than 0.05) lower (36.5 +/- 1.3 degrees C) compared with the normal subjects (38.0 +/- 0.2 degrees C).(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
989.
GRSL lymphoma cells were isolated from various growth sites in the host. The relative membrane lipid fluidities of these cells and of normal lymphoid cells were estimated by fluorescence polarization, using the probe diphenylhexatriene and by measuring the (free) cholesterol/phospholipid molar ratio in whole cells. The results indicate that the membrane fluidity (reciprocal of the lipid structural order) of the lymphoma cells increases in the order of their location: peripheral blood less than spleen less than mesenterial lymph node less than ascites fluid. The membrane fluidities of normal lymphocytes from thymus, mesenterial lymph node and spleen were about the same, but higher than of peripheral blood lymphocytes, and between those of the lymphoma cells from lymph node and spleen. These results are confirmed by more extensive analysis on purified plasma membranes from the splenic and ascitic GRSL lymphoma cells and from normal splenocytes and thymocytes. The significantly higher lipid order parameter found in the GRSL plasma membrane isolated from the spleen as compared to those from the ascites cells could be fully explained by the differences measured in the major chemical determinants of the fluidity, i.e., the cholesterol/phospholipid ratio, the sphingomyelin content and the degree of saturation of the fatty acyl groups of the phospholipids. It was also found that the cholesterol/phospholipid ratio in erythrocyte membranes isolated from the peripheral blood of the tumor bearers was higher than in those from normal control mice. The observed differences in membrane fluidity between distinct subsets of tumor cells may be relevant to the sensitivity of these cells to immune attack or to drugs. 相似文献
990.
G. G. Stott N. H. Mcarthur R. Tarpley R. F. Sis V. Jacobs 《Journal of fish biology》1980,17(5):593-602
Testes, collected from 11 species of demersal and platform associated pelagic fishes, were examined microscopically for petroleum induced lesions. The fish were taken from 19 oil and gas production platforms and four control sites from the north-central Gulf of Mexico, during two periods, spring and late summer. The status of the spermatogenic cycle at the time specimens were collected correlated well with the known spawning times for the respective species of fish with one exception. Histopathological evaluation revealed acidophilic granular cells, chromatophores, degeneration and focal leucocytic aggregations as the most prominent and abundant conditions in testes, with encapsulated cysts, protozoa and helminths being observed only rarely. No evidence of adverse effects on gonadal morphology was found. 相似文献